U.S. Sen. Peter Fitzgerald, whose Illinois seat is considered key to Republican chances to hold the Senate in 2004, will not seek re-election, Illinois Republicans said Tuesday.

Fitzgerald called close supporters Monday night to inform them of his decision not to seek a second term, said a source close to Fitzgerald who spoke only on condition of anonymity. The source, who was among those called by Fitzgerald, said the senator was to announce his decision Tuesday.

Neither Fitzgerald nor his aides returned repeated phone calls Tuesday. Aides at his Washington office declined to comment.

State Sen. Dave Syverson, treasurer of the Illinois Republican Party and a member of Fitzgerald's campaign finance committee, said people "very close" to Fitzgerald confirmed he will not seek re-election.

Illinois Treasurer Judy Baar Topinka, who heads the state GOP, said she had not spoken to Fitzgerald but had already contacted the White House to discuss the race.

"I've talked to the White House this morning and we are going to work together to find the best and strongest candidate available," Topinka said. "That will take a little bit of time. I don't think anybody was prepared for this."

Fitzgerald, who had frequent disputes with House Speaker Dennis Hastert and other Illinois Republicans, faced a potentially tough race in a state that has increasingly leaned Democratic in recent elections. Strategists in both parties view him as the most vulnerable Senate Republican up for re-election, and Fitzgerald had begun calling himself "the 51st senator" in recognition of how pivotal his re-election could be for preserving the GOP's Senate majority.

The Chicago Tribune, citing unnamed sources, reported in Tuesday's editions that the former state senator from Inverness told close associates he had "no fire in the belly" and cited concerns about the personal cost of a re-election campaign.

Fitzgerald, a little-known conservative state senator from Chicago's suburbs and heir to a banking fortune, spent $16 million of his own money in 1998 to unseat -- barely -- Democrat Carol Moseley Braun, the first black woman elected to the U.S. Senate.

A 2004 race was likely to be more expensive.

There has been grumbling within the state Republican party about Fitzgerald's performance over a host of issues, including the maverick's reluctance to fight for funding for Illinois projects that he considered political pork. He was likely to face primary opposition next March.

President Bush's political aides have insisted that Bush supports Fitzgerald.

Chicago-area businessman Andrew J. McKenna Jr. had already indicated he might be a primary challenger.

Democrats began lining up as early as 2002. They include former Chicago school board president Gery Chico, multimillionaire businessman Blair Hull, state Comptroller Dan Hynes, state Sen. Barack Obama, Cook County Treasurer Maria Pappas, businesswoman Joyce Washington and Metamora Mayor Matt O'Shea, a Republican who says he will run as a Democrat.